Many drugs such as diltiazem are metabolized by CYP3A4, the cytochrome P450 enzyme responsible for the metabolism of erythromycin and when given together, the possibility exists that one drug inhibits the metabolism of the other leading to an increase in plasma concentration. The magnitude of this interaction is not known. It is therefore important to define this effect because diltiazem is a drug that is used commonly to treat hypertension, angina and atrial fibrillation and these patients may be given erythromycin for the treatment of intercurrent bacterial infections.